The Bush administration yesterday admitted the federal budget deficit this year would be 50% higher than previous forecasts as it emerged that the Iraq war had so far cost the Pentagon $48bn (£30bn).

The White House said the US would end the year with a record $455bn budget shortfall, up from previous estimates of $304bn. Over six years, the deficit is now forecast to reach $1.9 trillion, up from an earlier estimate of $1.4 trillion.

The admission came a day after President George Bush, whose malapropism has spawned dozens of websites and books, uttered one of his most extraordinary gaffes yet when he told reporters that the US had invaded Iraq because Saddam Hussein had refused to allow weapons inspectors in.

Pressed during an Oval Office press conference on the now infamous assertion that Iraq had sought to buy uranium ore in Africa, he said: "The larger point is, and the fundamental question is, did Saddam Hussein have a weapons programme? And the answer is, absolutely. And we gave him a chance to allow the inspectors in, and he wouldn't let them in.

"And, therefore, after a reasonable request, we decided to remove him from power, along with other nations, so as to make sure he was not a threat to the United States and our friends and allies in the region."

The comment, made on Monday, received little coverage in the US media. Although it did raise some eyebrows at the Washington Post, which, in understated style noted: "The president's assertion that the war began because Iraq did not admit inspectors appeared to contradict the events leading up to the war this spring."

Saddam had in fact allowed weapons inspectors back in to Iraq on November 27 last year for the first time in four years and they remained there, albeit in a largely fruitless search, until the US and Britain decided to invade.

The White House did not re turn calls seeking comment.

The administration blamed the growing red ink on the weakness in the economy and the costs of fighting terrorism. The influential federal reserve chairman, Alan Greenspan, also raised concerns about the size of the deficit and called on the administration to exercise some "fiscal responsibility".

In response the White House said: "The tax cuts proposed by the president are not the problem. They are part of the solution." It believes the stimulus package will help cut the deficit to $226bn by 2008.

In an interview with the Associated Press, Dov Zakheim, the US defence department comptroller, said the military's role in Iraq now was costing an average of about $3.9bn a month. The Pentagon was given an additional $62.6bn this year, on top of its $364bn budget, to fight the war.

Mr Zakheim said he expected that all but $4bn of the additional cash would be spent before the end of September, the Pentagon's fiscal year.